1. Field of the Invention
The present invention generally relates to a driving method, and more particularly, to a method for driving pixels of a display panel.
2. Description of Related Art
FIG. 1 is a diagram illustrating a structure of a pixel which comprises a pixel structure with Cst on common. In FIG. 1, label 101 and label 102 represent a Nth gate line and a (N−1)th gate line in a display panel, respectively, wherein N is a natural number. And label 103 and label 104 all represent a source line, and label 105 represents a thin film transistor (TFT), and label 106 represents a pixel electrode, and label 107 represents a common line. In the structure, a portion of the pixel electrode 106 is overlapped with a portion of the common line 107 so as to form a storage capacitor Cst as shown in FIG. 2. FIG. 2 is a schematic diagram illustrating a structure in which the portion of the pixel electrode 106 is overlapped with the portion of the common line 107. Accordingly, the structure as shown in FIG. 1 is a pixel structure with Cst on common.
The pixel structure with Cst on common is widely used in various small-size display panels, and in the pixel structure, the common voltage modulation means that a modulation voltage source is used as a common voltage to drive the pixel, so as to lower the output voltage range of a source driver and decrease the cost of the source driver. However, because the pixel structure comprises the common line, an aperture ratio of the structure is smaller such that resolution, image quality and power consumption of the display panel are all undesired. Therefore, a display panel which comprises a pixel structure with a large aperture ratio to overcome the shortcomings of the conventional display panel is desired. However, when the display panel is fabricated by the pixel structure with a large aperture ratio, and the modulation voltage source is used as the common voltage to drive the pixel, when using the conventional gate line driving method (i.e. a pulse is provided to gate lines in turn, so as to turn on pixels coupled to the gate lines in turn), the quality of the image is decreased because of the inconsistent brightness shown when the pixel is turned off. The cause will be described as follows.
FIG. 3 is a diagram illustrating another structure of a pixel which comprises a pixel structure with Cst on gate. In FIG. 3, label 301 and label 302 represent a Nth gate line and a (N−1)th gate line in a display panel, respectively. And label 303 and label 304 represent two adjacent source lines, and label 305 represents a thin film transistor (TFT), and label 306 represents a pixel electrode. In the structure, there is no a common line, such that the aperture ratio of the structure is higher. In addition, in the structure, a portion of the pixel electrode 306 is overlapped with a portion of the gate line 302 so as to form a storage capacitor Cst as shown in FIG. 4. FIG. 4 is a schematic diagram illustrating a structure in which the portion of the pixel electrode 306 is overlapped with the portion of the gate line 302. Accordingly, the structure as shown in FIG. 3 is a pixel structure with Cst on gate. The pixel structure of Cst on gate have a higher aperture ratio than that of Cst on common. However, the Cst on gate structure with common voltage modulation Vcom results in the inconsistent brightness between periods of common voltage high and common voltage low. The cause will be described as follows.
FIG. 5 is a diagram illustrating an equivalent circuit of the structure as shown in FIG. 3. In FIG. 5, the labels 301-305 represent the corresponding elements as shown in FIG. 3, and Vcom represents a common voltage formed by a modulation voltage source (i.e. a potential of a common electrode of the substrate opposite to the TFT array substrate, hereinafter, called as a modulation common voltage Vcom), and Vp represents a voltage on the pixel electrode 306, and Clc represents a capacitor composed of the pixel electrode 306, the common electrode (not shown) and the liquid crystal layer between the pixel electrode 306 and the common electrode.
FIG. 6 is a diagram illustrating a signal waveform when the circuit as shown in FIG. 5 is used in the Kth image, wherein K is a natural number. Refer to FIG. 5 and FIG. 6, when a pulse 601 is provided to the gate line 301 so as to turn on the TFT 305, because a voltage of data loaded into the liquid capacitor Clc through the source line 303 is larger than a voltage of the modulation common voltage Vcom, during the TFT 305 is turned on, a level of the voltage Vp is pulled up, such that the brightness of the pixel is shown according to the voltage difference between the modulation common voltage Vcom and the voltage Vp. However, when the pulse 601 is turned to a low voltage from a high voltage, the TFT 305 is turn off to float the pixel electrode 306, such that the voltage Vp is vary because the variation of the modulation common voltage Vcom couples to the pixel electrode through the storage capacitor Cst. And in theory, the voltage Vp varies as shown in the line 603.
As shown in the dotted line 602 which represents the variation of the voltage Vp, even if the TFT 305 is turned off, if the voltage difference between the voltage Vcom and the voltage Vp is still unchanged, the brightness of the pixel will be fixed. However, in this case, a ΔVp of the voltage Vp is smaller than a ΔVcom of Vcom, such that the voltage Vp may mostly be pulled up to the level shown as label 603 in FIG. 6. Accordingly, when the pixel is turned off, the brightness is inconsistent between periods of common voltage high and common voltage low to decrease the average brightness which may be perceived by human eye.
FIG. 7 is a diagram illustrating a signal waveform when the circuit as shown in FIG. 5 is used in the (K+1)th image. Refer to FIG. 5 and FIG. 7, when a pulse 701 is provided to the gate line 301 so as to turn on the TFT 305, because a voltage of data loaded into the liquid capacitor Clc through the source line 303 is smaller than a voltage of the modulation common voltage Vcom, during the TFT 305 is turned on, a level of the voltage Vp is pulled down. However, when the TFT 305 is turned off to float the pixel electrode 306, such that the voltage Vp is vary because the variation of modulation common voltage Vcom couples to the pixel electrode through the storage capacitor Cst. And in theory, the voltage Vp varies as shown by a dotted line 702, however, in practice, the voltage Vp is pulled down to the level shown as label 703. Accordingly, when the pixel is turned off, the brightness is inconsistent between periods of common voltage high and common voltage low to decrease the average brightness which may be perceived by the human eye. Accordingly, although the pixel structure with Cst on gate has a higher aperture ratio, the brightness of the pixel is still decreased when the pixel is driven by the modulation common voltage Vcom.
FIG. 8 is a diagram illustrating another structure of a pixel. In FIG. 8, label 801 and label 802 represent a Nth gate line and a (N−1)th gate line, respectively. And label 803 and label 804 represent two adjacent source lines, and label 805 represents a thin film transistor (TFT), and label 806 represents a pixel electrode. In the structure, a portion of the pixel electrode 806 is overlapped with a portion of the gate line 802, a portion of the source line 803 and a portion of the source line 804 (the structure may increase the aperture ratio of the pixel) to form a parasitic capacitor Cg1, a parasitic capacitor Cd1 and a parasitic capacitor Cd2 besides a storage capacitor Cst. FIG. 9 is a diagram illustrating an equivalent circuit of the structure shown in FIG. 8. In FIG. 9, the labels 801-805, Cst, Cg1, Cd1 and Cd2 represent the corresponding elements as shown in FIG. 8, and Vcom represents a modulation common voltage, and Vp represents a voltage on the pixel electrode 806, and Clc represents a liquid capacitor between the pixel electrode 806 and the common electrode (not shown).
As described above, in the structure as shown in FIG. 8, after the TFT is turned off, because of the parasitic capacitors, the voltage variation value ΔVp of the voltage Vp is also not equal to the voltage variation value ΔVcom of the modulation common voltage Vcom. Accordingly, when the pixel is turned off, the brightness is inconsistent to decrease the average brightness which may be perceived by human eye.
FIG. 10 is a diagram illustrating another structure of a pixel, and the structure is one of the above-said specific structures. In FIG. 10, numerals 1001 and 1002 represent a Nth gate line and a (N−1)th gate line, respectively. And numerals 1003 and 1004 represent two adjacent source lines, and numeral 1005 represents a thin film transistor (TFT), and numeral 1006 represents a pixel electrode, and numeral 1007 represents a common line on the substrate of the TFT. In the structure, a portion of the pixel electrode 1006 is overlapped with a portion of the gate line 1001, a portion of the gate line 1002, a portion of the source line 1003 and a portion of the source line 1004 (the structure may increase the aperture ratio of the pixel) to form a parasitic capacitor Cg1, a storage capacitor Cst1, a parasitic capacitor Cd1 and a parasitic capacitor Cd2. In addition, a portion of the pixel electrode 1006 is also overlapped with a portion of the common line 1007 to form a storage capacitor Cst2 as shown in FIG. 11. FIG. 11 is a schematic diagram illustrating a structure in which the portion of the pixel electrode 1006 is overlapped with the portion of the common line 1007.
FIG. 12 is a diagram illustrating an equivalent circuit of the structure shown in FIG. 10. In FIG. 12, the numerals 1001-1007, Cg1, Cst1, Cd1 and Cd2 represent the corresponding elements as shown in FIG. 10, and Vcom represents a modulation common voltage, and Vp represents a voltage on the pixel electrode 1006, and Clc represents a liquid capacitor between the pixel electrode 1006 and the common electrode (not shown). In addition, the storage capacitor Cst2 is also described in FIG. 12. As described above, in the structure as shown in FIG. 10, after the TFT is turned off, because of these parasitic capacitors, the voltage variation value ΔVp of the voltage Vp is also not equal to the voltage variation value ΔVcom of the modulation common voltage Vcom. The voltage difference between Vp and Vcom don't keep a constant value. Accordingly, the brightness when the pixel is turned on and after the pixel is turned off may be inconsistent to decrease the average brightness which may be perceived by the human eye.